This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The objective of this study is to measure germ cell development of the ES cells and their ability to complete meiosis within the seminiferous tubules after transplantation into nonhuman primate testis. Specifically we will first conduct a series of model development studies designed to establish the feasibility of down-regulating spermatogenesis in monkeys with testosterone implants and eliminating spermatogonial stem cell populations with orally administered bisulfan exposure. Next, the left testis will receive populations of human ES cells that have not been subjected to in vitro differentiation while the right testis will receive cells exposed to 14 days of differentiation.